The bovine papillomaviruses are capable of transforming certain mouse fibroblast lines as well as certain rat fibroblast lines. The viral DNA in these transformed lines is maintained exclusively as extrachromosomal plasmids. The extrachromosomal nature of the viral DNA in these lines together with the selected malignant phenotypes has been utilized to develop the papillomaviruses as eukaryotic cloning vectors. We have cloned the rat preproinsulin gene together with the transforming region of the bovine papillomavirus and assessed the ability of the papillomaviruses to be used as cloning vectors. Recently, we have shown that the complete genome cloned into a deletion derivative of pBR322 (pML2) is capable of serving as a shuttle vector between bacteria and eukaryotic cells.